Add a "libvirt_disk_prefix" flag to libvirt driver
Add a "libvirt_disk_prefix" flag to libvirt driver

This change makes possible to override the default the prefix
that should be used in a particular installation (refers to
lp question 176978).

Change-Id: I06b9e7251dc4a02ef6ac40393dce9c5755b00614

n0ano/ganttclient